Electrical isolation barriers can be identified in many industrial, medical and communication applications where it is necessary to electrically isolate one section of electronic circuitry from another electronic section. In this context isolation exists between two sections of electronic circuitry if a large magnitude voltage source, typically on the order of one thousand volts or more, connected between any two circuit nodes separated by the barrier causes less than a minimal amount of current flow, typically on the order of ten milliamperes or less, through the voltage source. An electrical isolation barrier must exist, for example, in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. Specifically, in order to achieve regulatory compliance with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts rms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
In one particular example, Direct Access Arrangement (DAA) circuitry may be used to terminate the telephone connections at a phone line user's end to provide a communication path for signals to and from the phone lines. DAA circuitry includes the necessary circuitry to terminate the telephone connections at the user's end and may include, for example, an isolation barrier, DC termination circuitry, AC termination circuitry, ring detection circuitry, and processing circuitry that provides a communication path for signals to and from the phone lines. It is also desirable that the DAA circuitry act as an isolation barrier to meet the requirements of FCC regulations, Part 68. Examples of DAA circuitry known in the art may be found described in U.S. Pat. No. 6,198,816, U.S. Pat. No. 6,298,133, and U.S. Pat. No. 6,385,235, the disclosure of each being incorporated herein by reference. The DAA circuitry may utilize capacitors, transformers, opto-couplers, or a combination thereof for the isolation elements that provide the necessary phone line isolation.
In applications such as DAA circuitry there exists an analog or continuous time varying signal on one side of the isolation barrier, and the information contained in that signal must be communicated across the isolation barrier. For example, common telephone network modulator/demodulator, or modem, circuitry powered by a residential wall outlet must typically transfer an analog signal with bandwidth of approximately 4 kilohertz across an isolation barrier for transmission over the two-wire, public switched telephone network. It is usually desirable that the isolation method and associated circuitry provide this communication reliably and inexpensively. In this context, to achieve reliable transfer of information across the isolation barrier it is typically desirable that the following conditions apply: the isolating elements themselves do not significantly distort the signal information, the communication is substantially insensitive to or undisturbed by voltage signals and impedances that exist between the isolated circuitry sections and, finally, the communication is substantially insensitive to or undisturbed by noise sources in physical proximity to the isolating elements.
For example, single bit errors may result when common mode transients, also called electronic fast transients (EFTs), are present during transmission of information across the isolation elements of the isolation barrier. Typically, the information bits transmitted across the isolation barrier may include data bits, control bits and framing bits as discussed in U.S. Pat. No. 6,408,038 that describes a system for transmitting information across a capacitive barrier, the disclosure of which is incorporated herein by reference. Bits errors involving control bits may often be most troublesome as improperly interpreted control signals may change major system functions such as, for example, setting the system in the off-hook or on-hook state. One technique for alleviating the effects of EFTs is to transmit every control bit multiple times over a period of time. Thus, in one prior art example a control bit must be the same four times in a row before it takes effect. Since a control bit may be sent roughly every 64 usec., a single EFT (which is on the order of about 1 usec.) cannot corrupt more than one instance of the control bit.
Data bits may be provided at significantly higher rates, such as for example, 488 nsec/bit. Thus, a single EFT event can impact multiple data bits. Though individual data bits may not be as critical as control bits for system operations, the throughput of a modem may be reduced due to data bit errors. For example, EFTs during a modem connection may result in decreased modem throughput.
Thus, it would be desirable to implement a DAA circuit that has a cost effective and efficient solution to EFTs and other error sources that may impact data bits.